The 10-second attack was so brief that a group of people standing nearby boarded a bus and left, unaware of the tragedy that had just unfolded, according to witnesses.

Eighteen years after the killing of the black teenager, two men are standing trial for murder. Gary Dobson, 36, and David Norris, 35, both deny being part of the gang of youths that surrounded the 18-year-old in an unprovoked attack, leaving him with two fatal stab wounds.

The new case hinges on tiny specks of blood and fibres discovered by forensic scientists as part of a cold case review in 2007 – findings that the two men's defence teams have dismissed as less than "a teaspoon" of evidence generated by cross-contamination and police tampering.

Yesterday some of the last people to see Stephen alive on the night of 22 April 1993 took the stand. Hospital worker, Royston Westbrook, explained how he was on his way home after a shift finishing at 10pm when a rumoured strike convinced him, along with Stephen and his friend Duwayne Brooks, 18, to take another bus, stopping at the spot in Eltham, south London, that would become the scene of the crime minutes later.

The two teenagers were chatting about football but, fed up with waiting, decided to walk to a nearby roundabout to see if a bus was coming. A few seconds later they were running back, Mr Westbrook said, pursued by a gang of white youths which had emerged from the other side of the road.

He said: "They just collided, that's what it looked like. That's when they surrounded the pair of them. They grabbed Duwayne Brooks's wrist. "He just turned his wrist and pulled away. Duwayne ran towards the bus stop we were standing at, shouting something like, 'Leg it, Stephen' or 'Run, Stephen'."